A heat pump reverses its cycle to pull heat out of your house

A heat pump works as an air conditioner by running its refrigerant cycle in reverse. Instead of moving heat from outside air into your home (as it does in heating mode), it pulls heat from inside your house and pushes it outdoors. The same compressor, coils, and refrigerant that warm your home in winter do the cooling work in summer—the system just changes direction.

When you set your thermostat to cooling mode, a valve called the reversing valve switches the flow of refrigerant. The indoor coil (called the evaporator in cooling mode) becomes cold and absorbs heat from the air inside your home. That heat travels through the refrigerant to the outdoor coil (now called the condenser), where it is released outside. A fan blows indoor air across the cold coil, and cooled air flows back into your rooms.

The result feels identical to a traditional air conditioner. You get the same temperature control, the same dehumidification (the cold coil removes moisture from the air), and the same comfort. The difference is that a heat pump does both heating and cooling with one unit, while a traditional AC system only cools.

Key Takeaways

  • A heat pump cools by reversing its refrigerant flow so the indoor coil absorbs heat instead of releasing it.
  • The reversing valve is the component that switches the system between heating and cooling modes.
  • Cooling performance depends on the outdoor temperature—heat pumps cool less efficiently when it is very hot outside.
  • Most heat pumps include a backup electric heating element that activates if the outdoor temperature drops below the system's effective range.
  • Heat pump cooling costs less to run than traditional AC in moderate climates because the system uses the same equipment for both seasons.

The refrigerant cycle that makes cooling happen

Refrigerant is a chemical that changes between liquid and gas at low temperatures and pressures. In cooling mode, the compressor pressurizes the refrigerant gas, which heats it up. This hot gas flows to the outdoor coil, where a fan cools it down and it condenses into a liquid. As the liquid refrigerant cools, it releases the heat it absorbed from inside your home—that heat blows away outdoors.

The liquid refrigerant then flows through an expansion device (a metering valve or capillary tube) that reduces its pressure. This sudden drop in pressure causes the refrigerant to evaporate into a gas again, and evaporation is a process that absorbs heat. The cold gas flows to the indoor coil, where it absorbs heat from the air in your home. A blower fan pushes your indoor air across this cold coil, cooling it down before it returns to your rooms.

The cycle repeats continuously as long as cooling is needed. The compressor runs, pressurizes the gas, and the refrigerant loops between the indoor and outdoor coils, always moving heat in the same direction: out of your house and into the outside air.

Why outdoor temperature affects cooling power

A heat pump's cooling capacity depends on how much heat it can reject to the outdoor air. On a mild 75-degree day, the outdoor coil can easily release heat to air that is cooler than the refrigerant. On a 95-degree day, the temperature difference is smaller, so the coil has to work harder and the system cools more slowly. On extremely hot days (above 105 degrees), some heat pumps struggle to cool effectively because the outdoor air is nearly as warm as the refrigerant itself.

This is different from a traditional air conditioner, which also loses efficiency in extreme heat but is designed specifically for cooling and often performs better in very hot climates. If you live in a region with frequent temperatures above 100 degrees, ask your installer whether a heat pump or a traditional AC system is the better choice for your area.

Most heat pumps are rated for cooling down to outdoor temperatures around 115 degrees Fahrenheit, though real-world performance declines as you approach that limit. Manufacturers publish cooling capacity ratings (measured in BTU per hour) at standard conditions, usually 95 degrees outside and 80 degrees inside.

How the reversing valve switches between heating and cooling

The reversing valve is a small component that looks like a cylinder with ports on each end. Inside it is a sliding piston that directs refrigerant flow. When you switch your thermostat from heating to cooling, an electrical signal energizes a solenoid on the valve, which moves the piston and reverses the path the refrigerant takes through the system.

In heating mode, the outdoor coil absorbs heat from outside air and the indoor coil releases it into your home. In cooling mode, those roles flip: the indoor coil absorbs heat and the outdoor coil releases it. The same compressor, the same refrigerant, and the same coils do both jobs—only the direction changes.

If the reversing valve fails or gets stuck, the system may blow warm air when you want cooling, or cold air when you want heat. A stuck valve is one of the most common heat pump repairs. If your system is not switching modes correctly, a technician can test the valve with a multimeter and replace it if needed.

Dehumidification: removing moisture while cooling

When warm, humid air passes over the cold indoor coil, moisture condenses on the coil surface—the same way water beads up on a cold window. This condensation drips into a pan and flows out through a drain line to the outside or to a floor drain. As a result, a heat pump cools and dehumidifies at the same time, which is why the air feels more comfortable than just lowering the temperature would.

The drain line can become clogged with algae or debris, which backs up water into the pan and can damage the system or create a musty smell. Most heat pumps have a safety switch that shuts the system down if water backs up too far. If you notice water pooling near your indoor unit or smell mold, the drain line likely needs cleaning. This is a task a technician can handle quickly, or in some cases you can clear a clogged line yourself by flushing it with a wet/dry vacuum.

Backup heating when cooling mode is not enough

Many heat pumps include an electric resistance heating element (called auxiliary heat or emergency heat) that activates in cooling mode if the system cannot keep up. This happens rarely—usually only during a power surge or if the compressor fails—but the element is there as a safety net. In heating mode, auxiliary heat kicks in automatically when outdoor temperatures drop below the heat pump's effective range, typically around 35 to 40 degrees Fahrenheit, depending on the model.

You should never manually switch to "emergency heat" mode unless a technician tells you to. Emergency heat uses electric resistance, which is much more expensive to run than the heat pump cycle. If your system is running on emergency heat for more than a few hours, something is wrong and you should call for service.

Comparing heat pump cooling to traditional air conditioning

A traditional central air conditioner cools only. It has a compressor and coils designed specifically for that job, and it does not need a reversing valve. In moderate climates, a heat pump costs less to operate because you get both heating and cooling from one system, so you are not paying for two separate units. In very hot climates, a traditional AC may cool faster and more reliably, especially on days above 100 degrees.

Heat pumps are becoming more common in hot regions as technology improves. Modern cold-climate heat pumps can heat effectively even when outdoor temperatures are well below freezing. If you are replacing an old AC system, a heat pump is worth considering even in a warm area, because it eliminates the need for a separate heating system and reduces your overall energy use across the year.

The choice depends on your climate, your heating needs, and your budget. A technician can help you compare the operating costs of each option based on your local electricity rates and typical weather patterns.

Frequently Asked Questions

Can a heat pump cool as well as a window air conditioner?

A central heat pump cools a whole house, while a window unit cools one room. For that single room, a window AC may cool faster because it is smaller and more focused. For whole-home cooling, a properly sized heat pump performs as well as a central air conditioner in moderate climates. In very hot climates, traditional AC sometimes has an edge.

Why does my heat pump make noise when switching to cooling mode?

The reversing valve produces a brief hissing or clicking sound when it switches direction. This is normal and lasts only a few seconds. If the noise is loud, continuous, or accompanied by warm air when you want cooling, the valve may be stuck and needs service.

Does a heat pump cool as fast as it heats?

Cooling and heating cycles are similar, but cooling often feels faster because people notice temperature changes more when the air is cold. Both modes depend on outdoor conditions and the size of your home. If cooling seems slow, check that your thermostat is set to cooling mode and that filters are clean.

What happens to a heat pump in extreme heat?

Most heat pumps cool less efficiently when outdoor temperatures exceed 95 degrees, and some struggle above 105 degrees. The system keeps running but moves heat more slowly. If you live in a very hot climate, ask your installer about high-temperature heat pump models, which are designed to handle extreme conditions better than standard units.

Can I use a heat pump to cool if I do not need heating?

Yes. You can run a heat pump in cooling-only mode year-round if you have a separate heating system or live in a climate where you do not need heat. However, you would be paying for heating capability you do not use, so a traditional air conditioner might be more cost-effective in that situation.